Method for joining metallic members, joint structure and brazing filler metal

ABSTRACT

In joining an Fe-based metallic member comprising an Fe-based material and an Al-based metallic member comprising an Al-based material by a Zn-based brazing filler metal, a joined part of the Fe-based metallic member is heated at a temperature higher than a melting point of the Fe-based material.

The present application is a continuation of U.S. application Ser. No.12/933,578, which entered the U.S. National Stage on Sep. 20, 2010 fromPCT Application No. PCT/JP2009/057928, filed on Apr. 21, 2009. Thecontents of the parent application are hereby incorporated in full byreference. The present invention relates to a method for joining aFe-based metallic member and an Al-based metallic member by interposinga brazing filler metal between the Fe-based metallic member and theAl-based metallic member, a joint structure, and a brazing filler metal.

TECHNICAL FIELD Background Art

Joint structure of metallic members, such as various joints, is producedby joining dissimilar metallic members. In the joining of dissimilarmetallic members, brazing is conducted by irradiating a brazing fillermetal interposed between those dissimilar metallic members with laserbeam, and heating the brazing filler metal. A joining layer is formedthereby between the dissimilar metallic members. Thus, a joint structureof metallic members is produced.

For example, in the case of using a Fe-based metallic member containinga Fe-based material and an Al-based metallic member containing anAl-based material as dissimilar metallic members, Al and Zn do not forma compound layer and forms an eutectic structure in a wide range. Forthis reason, a Zn-based brazing filler metal is used as a brazing fillermetal. This can ensure strength between the Al-based metallic member andthe joining layer.

In order to suppress growth of a reaction layer (for example,intermetallic compound layer) formed in the interface part between theFe-based metallic member and the joining layer, it is effective todecrease a reaction temperature and shorten a reaction time. For thisreason, Al which decreases a melting point of a brazing filler metal byforming an eutectic alloy together with Zn is used as an additiveelement (for example, see Patent Document 1).

However, in the case that an intermetallic compound layer formed in theinterface part between the Fe-based metallic member and the joininglayer is brittle, the breakage may be generated in such theintermetallic compound layer. As a result, strength of a joint structureof dissimilar metallic members was considerably decreased as comparedwith that of a joint structure of similar metallic members.

When joining is conducted by laser irradiation at low temperatures, heatinput into a joined part of dissimilar metallic members is conducted bythermal conduction from a surface of the brazing filler metal.Therefore, in the interface part of the joined part, thermal history isdifferent every site. For this reason, in the interface part of thejoined part, a reaction layer grows heterogeneously, thereby partiallyforming an unreacted layer and increasing a thickness of the reactionlayer. As a result, joint strength was decreased. Particularly, in orderto obtain good joining in the joining of dissimilar metallic members, arange of a joining temperature is limited to a predetermined range,unlike similar metallic members. Therefore, the problem due to thethermal history was serious.

PRIOR ART REFERENCES Patent Document

Patent Document 1: Japanese Patent No. 3740858

SUMMARY OF THE INVENTION

One or more examples of the present invention provide a method forjoining metallic members and a joint structure. The method can improvejoint strength between a Fe-based metallic member and an Al-basedmetallic member by increasing joint strength in the interface partsbetween the Fe-based metallic member and a joining layer.

The present inventors made keen investigations to heating technology atthe joining of a Fe-based metallic member and an Al-based metallicmember using a Zn-based brazing filler metal. In the conventionaljoining using the Zn-based brazing filler metal, only the Zn-basedfiller metal was heated so as not to melt the Fe-based metallic member.As a result of investigations to the conventional joining, the presentinventors have found that joint strength between the Fe-based metallicmember and the joining layer having a Zn-based brazing filler metal canbe increased by melting the joined part of the Fe-based metallic memberby heating at a temperature higher than a melting point of the Fe-basedmaterial.

According to one or more examples of the present invention, in a methodfor joining the Fe-based metallic member containing the Fe-basedmaterial and the Al-based metallic member containing the Al-basedmaterial by interposing the Zn-based brazing filler metal between theFe-based metallic member and the Al-based metallic member, the joinedpart of the Fe-based metallic member is heated at the temperature higherthan the melting point of the Fe-based material at the joining.

In the method for joining metallic members according to the aboveexamples, the joined part of the Fe-based metallic member containing theFe-based material is heated at the temperature higher than the meltingpoint of the Fe-based material at the joining. Therefore, an Al—Fe—Znsystem intermetallic compound layer containing Al as a main componentcan be formed in the interface part between the Fe-based metallic memberand the joining layer containing the Zn-based brazing filler metal. Theintermetallic compound layer has high ductility, so that joint strengthbetween the Fe-based metallic member and the joining layer can beincreased. Consequently, joint strength between the Fe-based metallicmember and the Al-based metallic member can be improved. Furthermore,since the joined part of the Fe-based metallic member is heated at thetemperature higher than the melting point of the Fe-based material asdescribed above, the Zn-based material and a Fe—Zn-based materialvaporize. By this, a plated portion plated on the Fe-based materialvaporizes regardless of the kinds of plating such as GA plating or GIplating, and as a result, good joint part can be obtained regardless ofthe kinds of plating. Furthermore, an oxide coating film on the surfaceof the Fe-based material is removed by vapor pressure in the melting andthe vaporization by overheating. Therefore, even though flux is notused, good joining of dissimilar materials can be conducted. The term“joined part” used herein means a predetermined joint part between theFe-based metallic member and the Al-based metallic member beforejoining, and the term “joint part” used herein means the predeterminedjoint part after joining.

Various constitutions can be used in the method for joining metallicmembers according to the above examples. For example, a groove shape isformed by the Fe-based metallic member and the Al-based metallic member,a Zn-based brazing filler metal is placed in the groove shape, and inthe joining, a center line of laser beam can be positioned at theFe-based metal member side relative to the center line of the grooveshape. In this embodiment, the Fe-based material of the Fe-basedmetallic member can selectively be melted. Therefore, an intermetalliccompound layer can be formed into a stable layer shape over the entireinterface part between the Fe-based metallic member and the joininglayer. Furthermore, since the Al-based material of the Al-based metallicmember is not excessively heated, the Al-based material can be preventedfrom dropping down. Consequently, joint strength between the Fe-basedmetallic member and the Al-based metallic member can further beimproved.

The joined part of the Fe-based metallic member can be heated such thata key hole is formed in the Fe-based metallic member at the joining. The“key hole” used herein means a hollow portion formed by melting ametallic member. In this embodiment, the Zn-based brazing filler metalflows into the melted portion of the Fe-based metallic member at thejoining, and therefore, a shape that the joining layer is fitted to theFe-based metallic member can be obtained. Furthermore, laser multiplyreflects in the key hole. Therefore, energy density is high, and thesurface temperature in the keyhole can be maintained uniformly. This canuniformly form a metal compound layer containing the Al—Fe—Zn systemintermetallic compound over the upper portion, the central portion andthe lower portion of the joint part. As a result, strength at the jointpart between the Fe-based metallic member and the joining layer canfurther be increased, and consequently, joint strength between theFe-based metallic member and the Ai-based metallic member can further beimproved.

According to one or more examples of the present invention, in the jointstructure in which the Fe-based metallic member containing the Fe-basedmaterial and the Al-based metallic member containing the Al-basedmaterial are joined by interposing the joining layer containing Zn as amain component therebetween, the joining layer contains Al, and anintermetallic compound layer containing an Al—Fe—Zn system intermetalliccompound, whose main component is Al, is formed in the interface partbetween the Fe-based metallic member and the joining layer. Variousconstitutions can be used in the joint structure of the metallic membersaccording to the above examples. For example, the joint structure canhave a shape in which the joining layer is fitted to the Fe-basedmetallic member.

According to the method for joining metallic members or the jointstructure according to the above examples, the intermetallic compoundlayer containing the Al—Fe—Zn-based intermetallic compound, whose maincomponent is Al, can be formed in the interface part between theFe-based metallic member and the joining layer containing the Zn-basedbrazing filler metal. Furthermore, the intermetallic compound layer hashigh ductility, and therefore, the effect that can increase jointstrength between the Fe-based metallic member and the joining layer canbe obtained.

One or more examples of the present invention provide a brazing fillermetal that can obtain strength of nearly the same degree as that of ajoint structure of similar metallic members in the joint structure ofdissimilar metallic members, and a method for joining metallic membersusing the brazing filler metal.

The present inventors made keen investigations to a brazing filler metalapplied to the joining of dissimilar metals using the Fe-based metallicmember and the Al-based metallic member as metallic members.Conventionally, in the case of adding Si to Zn, a melting point of theresulting material increases with the addition of Si, and becomes highas about 600 to 900° C., as shown in FIG. 18(A). For this reason, Si wasnot investigated as an additive element of a brazing filler metal. Anelement generally used as an additive element of the Zn-based brazingfiller metal is Al, which decreases a melting point of the brazingfiller metal by the formation of an eutectic alloy with Zn, as shown inFIG. 18(B). FIG. 18(A) is a Zn—Si system binary equilibrium diagram, andFIG. 18(B) is a Zn—Al system binary equilibrium diagram (Source: BinaryAlloy Phase Diagrams, ASM International, Materials Park).

On the other hand, the present inventors have found that anintermetallic compound layer is not formed in the interface part betweenthe Fe-based metallic member and the joining layer by using aZn—Si-based brazing filler metal containing Si as an additive element.

According to one or more examples of the present invention, the brazingfiller metal is a brazing filler metal used to join the Fe-basedmetallic member containing the Fe-based material and the Al-basedmetallic member containing an Al-based material, and comprises Zn, Siand unavoidable impurities.

The joining of dissimilar metallic members of the Fe-based metallicmember containing the Fe-based material and the Al-based metallic membercomprising an Al-based material is conducted by using the Zn—Si-basedbrazing filler metal containing Si as the additive element, andtherefore, an intermetallic compound layer is not formed in theinterface part between the Fe-based metallic member and the joininglayer. In the case that the intermetallic compound layer in theinterface part between the Fe-based metallic member and the joininglayer is brittle, the joint structure strength was decreased. However,such an intermetallic compound layer is not formed by using theZn—Si-based brazing filler metal, and therefore, strength of theinterface part between the Fe-based metallic member and the joininglayer can be increased. As a result, joint strength nearly the same asthat of the joining of similar metallic members can be obtained.

The brazing filler metal of the above examples can use variousconstitutions. For example, the brazing filler metal can contain 0.25 to2.5% by weight of Si, and the remainder being Zn and unavoidableimpurities. This embodiment can further improve joint strength(particularly, peel strength).

According to one or more examples of the present invention, the methodfor joining metallic members is a method for joining the Fe-basedmetallic member containing the Fe-based material and the Al-basedmetallic member containing the Al-based material by interposing thebrazing filler metal between the Fe-based metallic member and theAl-based metallic member. The brazing filler metal contains Zn, Si andunavoidable impurities.

According to one or more examples of the present invention, variousconstitutions can be used in the method for joining metallic members.For example, the Fe-based metallic member in the joined part can beheated at a temperature higher than a melting point of the Fe-basedmaterial. In this embodiment, the joined part of the Fe-based metallicmember contains the Fe-based material can be heated at a temperaturehigher than a melting point of the Fe-based material, and therefore, thekey hole can be formed in the joined part of the Fe-based metallicmember at the joining. The “key hole” used herein means a hollow portionformed by melting a metallic member. The term “joined part” used hereinmeans a predetermined joint part between the Fe-based metallic memberbefore joining and the Al-based metallic member, and the term “jointpart” used herein means a predetermined joint part after joining.

In the heating by laser irradiation, laser beam multiply reflects in thekeyhole. Therefore, energy density is increased in the key hole, and theentire surface from upper side to the lower side in the key hole isheated nearly uniformly. By this, after heating the joined part, amolten Zn—Si-based brazing filler metal entered the key hole canuniformly react with the entire surface in the key hole. Therefore,strength in the interface part between the Fe-based metallic member andthe joining layer can further be increased, and consequently, jointstrength of a joined structure can be further improved.

The Zn-based material and the Fe—Zn-based material vaporize. By this, aplated portion plated on the Fe-based material vaporizes regardless ofthe kinds of plating such as GA plating or GI plating, and as a result,good joint part can be obtained regardless of the kind of the plating.Furthermore, an oxide coating film on the surface of the Fe-basedmaterial is removed by vapor pressure in the melting and thevaporization due to overheating. Therefore, even though flux is notused, joining of good dissimilar metallic members can be conducted well.

According to the brazing filler metal or the method for joining metallicmembers using the same, according to one or examples of the presentinvention, a brittle intermetallic compound layer is not formed in theinterface part between the Fe-based metallic member and the joininglayer. Therefore, strength of the interface part between the Fe-basedmetallic member and the joining layer can be improved, and as a result,the resulting joined structure can obtain the effect that can obtainjoint strength as nearly the same as that in the joining of similarmetallic members.

One or more examples of the present invention provide a method forjoining metallic members, that can improve joint strength by preventinggeneration of thermal history every site in the interface part of thejoined part of dissimilar metallic members.

According to one or more examples of the present invention, in themethod for joining plural metallic members by the brazing filler metalusing laser beam as a heat source, the Fe-based metallic membercontaining the Fe-based material and the Al-based metallic membercontaining the Al-based material are used as metallic members, and theZn-based brazing filler metal is used as a brazing filler metal. In thejoining of the Fe-based metallic member and the Al-based metallicmember, the brazing filler metal is vaporized by the irradiation withlaser beam, the joined part of metallic members is melted to form a keyhole, and laser beam multiply reflects in the key hole. The “key hole”used herein means a hollow portion formed by melting a metallic member.The term “joined part” used herein means a predetermined joint partbetween the Fe-based metallic member and the Al-based metallic memberbefore joining, and the term “joint part” used herein means apredetermined joint part after joining.

In the joining method of the above examples, the brazing filler metal isvaporized by the irradiation of laser beam, the welded part of themetallic member is melted, and the key hole is formed. During heatingthe welded part, the vaporized brazing filler metal fills the key hole,and the remaining brazing filler metal is present on the periphery ofthe upper end portion of the key hole together with the molten metal.After heating the welded part, those molten materials enter the keyhole, and form a reaction layer.

During heating the welded part, the laser beam multiply reflects in thekey hole. Therefore, energy density is increased in the key hole, andthe entire surface from the upper side to the lower side in the key holeis nearly uniformly heated. By this, after heating the welded part, themolten material entering the key hole can uniformly react with theentire surface in the key hole. Furthermore, the joining at lowtemperature is possible, and the molten material entering the key holecan instantly be coagulated. As a result, the interface part between theFe-based metallic member and the joining layer can uniformly be cooled.

Therefore, in the case that the reaction layer is formed between theFe-based metallic member and the joining layer, the reaction layer hasan uniform layer shape, and joint strength can be improved. Furthermore,in the case that the reaction layer is not formed between the Fe-basedmetallic member and the joining layer, such a brittle layer is notpresent, and additionally unevenness is not generated in strengthdistribution in the interface part between the Fe-based metallic memberand the joining layer. As a result, joint strength can greatly beimproved.

When a key hole is formed, joining area is increased. Therefore, theabove effect can well be obtained. Zn-based plating and alloyedFe—Zn-based plating vaporize. In this case, plated portion plated on theFe-based material vaporizes regardless of the kinds of plating such asGA plating or GI plating. As a result, good joint part can be obtainedregardless of the kinds of plating. Furthermore, an oxide coating filmon the surface of the Fe-based material is removed by vapor pressure inthe melting and the vaporization due to overheating. Therefore, eventhough flux is not used, joining of dissimilar metallic members can wellbe conducted.

In the joining of dissimilar metallic members, the region of the joiningtemperature is limited to a predetermined region unlike the case ofsimilar metallic members. Therefore, the method for joining metallicmembers according to the above examples in which the entire surface fromthe upper side to the lower side in the key hole can uniformly be heatedis particularly effective to the joining of dissimilar metallic membersin which the region of the joining temperature is limited to thepredetermined region. The effect by this is remarkable as compared withthe conventional joining of dissimilar metallic members.

According to one or more examples of the present invention, in themethod for joining plural metallic members by the brazing filler metalusing laser beam as a heat source, the Fe-based metallic membercontaining the Fe-based material and the Al-based metallic membercontaining the Al-based material are used as the metallic members, theZn-based brazing filler metal is used as the brazing filler metal, and agroove shape is formed by the Fe-based metallic member and the Al-basedmetallic member. In the joining of the Fe-based metallic member and theAl-based metallic member, the brazing filler metal is vaporized by theirradiation with laser beam, and laser beam multiply reflects on thesurface of the groove shape.

In the method for joining metallic members according to the aboveexamples, in place of conducting multiple reflection of laser beam inthe key hole formed by melting the welded part of the metallic members,the welded parts of the metallic members are not melted, and multiplereflection of laser beam is conducted in the groove shape formed by theFe-based metallic member and the Al-based metallic member. By conductingmultiple reflection of laser beam on the surface of the groove shapeformed by the Fe-based metallic member and the Al-based metallic member,the entire surface of the groove shape can nearly uniformly be heated.As a result, the above effect by the multiple reflection can beobtained.

According to the method for joining metallic members according to theabove examples, the multiple reflection of laser beam is conducted inthe key hole or on the surface of the groove shape, and therefore, theentire surface can nearly uniformly be heated. As a result, the effectthat the joint strength in the interface part between the Fe-basedmetallic member and the joining layer can be improved can be obtained.

Other characteristic and effects are apparent from the description ofExamples and the claims attached.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) and FIG. 1(B) show the state of producing a joined structureby a method for joining metallic members according to a firstembodiment. FIG. 1(A) is a schematic perspective view, and FIG. 1(B) isa side view of a joined part.

FIG. 2(A) and FIG. 2(B) show the example of irradiation state of laserbeam to a joined part of metallic members in FIG. 1(A) and FIG. 1(B).FIG. 2(A) is an enlarged front view in the case that the center line oflaser beam consists with the center line of a groove shape of metallicmembers, and FIG. 2(B) is an enlarged front view in the case that thecenter line of laser beam shifts to the Fe-based metallic member sidefrom the center line of the groove shape of metallic members.

FIG. 3 is a block diagram showing the joined structure of metallicmembers according to the first embodiment.

FIGS. 4(A) to 4(D) are SEM photographs of the joined structure ofmetallic members of Example 1 according to the first embodiment. FIG.4(A) is a whole photograph of the joint part and its neighborhood, FIG.4(B) is a photograph of an upper portion in a joined interface partbetween an Fe-based metallic member and a joining layer, FIG. 4(C) is aphotograph of a central portion in a joined interface part between anFe-based metallic member and a joining layer, and FIG. 4(D) is aphotograph of a lower portion of a joined interface part between anFe-based metallic member and a joining layer.

FIGS. 5(A) to 5(D) are SEM photographs of the joined structure ofmetallic members of Example 2 according to the first embodiment. FIG.5(A) is a whole photograph of the joint part and its neighborhood, FIG.5(B) is a photograph of an upper portion of a joined interface partbetween an Fe-based metallic member and a joining layer, FIG. 5(C) is aphotograph of a central portion of a joined interface part between anFe-based metallic member and a joining layer, and FIG. 5(D) is aphotograph of a lower portion of a joined interface part between anFe-based metallic member and a joining layer.

FIGS. 6(A) to 6(D) are SEM photographs of the joined structure ofmetallic members of Example 3 according to the first embodiment. FIG.6(A) is a whole photograph of the joint part and its neighborhood, FIG.6(B) is a photograph of an upper portion of a joined interface partbetween an Fe-based metallic member and a joining layer, FIG. 6(C) is aphotograph of a central portion of a joined interface part between anFe-based metallic member and a joining layer, and FIG. 6(D) is aphotograph of a lower portion of a joined interface part between anFe-based metallic member and a joining layer.

FIGS. 7(A) to 7(D) are SEM photographs of the joined structure ofmetallic members of Comparative Example 1. FIG. 7(A) is a wholephotograph of the joint part and its neighborhood, FIG. 7(B) is aphotograph of an upper portion of a joined interface part between anFe-based metallic member and a joining layer, FIG. 7(C) is a photographof a central portion of a joined interface part between an Fe-basedmetallic member and a joining layer, and FIG. 7(D) is a photograph of alower portion of a joined interface part between an Fe-based metallicmember and a joining layer.

FIGS. 8(A) to 8(D) are SEM photographs of the joined structure ometallic members of Comparative Example 2. FIG. 8(A) is a wholephotograph of the joint part and its neighborhood, FIG. 8(B) is aphotograph of an upper portion of a joined interface part between anFe-based metallic member and a joining layer, FIG. 8(C) is a photographof a central portion of a joined interface part between an Fe-basedmetallic member and a joining layer, and FIG. 8(D) is a photograph of alower portion of a joined interface part between an Fe-based metallicmember and a joining layer.

FIGS. 9(A) to 9(D) are SEM photographs of the joined structure ofmetallic members of Comparative Example 3. FIG. 9(A) is a wholephotograph of the joint part and its neighborhood, FIG. 9(B) is aphotograph of an upper portion of a joined interface part between anFe-based metallic member and a joining layer, FIG. 9(C) is a photographof a central portion of a joined interface part between an Fe-basedmetallic member and a joining layer, and FIG. 9(D) is a photograph of alower portion of a joined interface part between an Fe-based metallicmember and a joining layer.

FIGS. 10(A) and 10(B) show a schematic constitution of the state ofproducing a joined structure by the method for joining metallic membersaccording a second embodiment. FIG. 10(A) is a schematic perspectiveview, and FIG. 10(B) is a side view.

FIG. 11 is a cross-sectional view showing a constitution of the joinedpart in which a key hole is formed at the joining of the metallicmembers according to the second embodiment.

FIG. 12 is a cross-sectional block diagram showing one example of thejoined structure obtained by the method for joining metallic membersaccording to the second embodiment.

FIG. 13(A) is SEM photograph (left-side photograph) of a joint part of ajoined structure and enlarged SEM photograph (right-side photograph) ofan interface between an Fe-based metallic member and a joining layer inthe left-hand photograph, and FIG. 13(B) is EPMA map analysisphotographs of the interface shown in the enlarged SEM photograph ofFIG. 13(A).

FIG. 14(A) and FIG. 14(B) are SEM photographs of an interface between anFe-based metallic member and a joining layer. FIG. 14(A) is SEMphotograph of 3,000 magnifications, and FIG. 14(B) is SEM photograph of15,000 magnifications.

FIGS. 15(A) and 15(B) are schematic cross-sectional views of a joinedstructure for explaining methods of a flare tensile strength test and apeel strength test.

FIG. 16 is a graph showing strength of each sample obtained in the flaretensile strength test.

FIG. 17 is a graph showing strength of each sample obtained in the peelstrength test.

FIG. 18(A) is a binary equilibrium diagram of ZnSi, and FIG. 18(B) is abinary equilibrium diagram of ZnAl.

FIGS. 19(A) and 19(B) show schematic constitutions of the state ofproducing a joined structure by a method for joining metallic membersaccording to a third embodiment. FIG. 19(A) is a perspective view, andFIG. 19(B) is a view seen from an Al-based metallic member in FIG.19(A).

FIGS. 20(A) to 20(D) show schematic constitutions of the method forjoining metallic members according to the third embodiment, and FIGS.20(A) to 20(D) are schematic views seen from the same direction as inFIG. 19(B) in each step.

FIGS. 21(A) to 21(D) show schematic constitutions of the method forjoining metallic members according to the third embodiment, and FIGS.21(A) to 21(D) are schematic views seen from the front of FIG. 19(A) ineach step.

FIGS. 22(A) and 22(B) are cross-sectional block diagrams showing oneexample of a joined structure obtained by the method for joiningmetallic members according to the third embodiment.

FIGS. 23(A) to 23(D) show schematic constitutions of a method forjoining metallic members according to a fourth embodiment, and FIGS.23(A) to 23(D) are schematic views seen from the front of FIG. 19(A) ineach step.

FIGS. 24(A) and 24(B) are cross-sectional block diagrams showing anexample of the joined structure obtained by the method for joiningmetallic members according to the fourth embodiment.

FIGS. 25(A) to 25(D) are SEM photographs of the joined structure ofmetallic members of the sample of Example 1. FIG. 25(A) is a wholephotograph of the joint part and its neighborhood,

FIG. 25(B) is a photograph of an upper portion of a joined interfacepart between an Fe-based metallic member and a joining layer, FIG. 25(C)is a photograph of a central portion of a joined interface part betweenan Fe-based metallic member and a joining layer, and FIG. 25(D) is aphotograph of a lower portion of a joined interface part between anFe-based metallic member and a joining layer.

FIGS. 26(A) to 26(D) are SEM photographs of a joined structure ofmetallic members of the comparative sample of Example 1. FIG. 26(A) is awhole photograph of the joint part and its neighborhood, FIG. 26(B) is aphotograph of an upper portion of a joined interface part between anFe-based metallic member and a joining layer, FIG. 26(C) is a photographof a central portion of a joined interface part between an Fe-basedmetallic member and a joining layer, and FIG. 26(D) is a photograph of alower portion of a joined interface part between an Fe-based metallicmember and a joining layer.

FIGS. 27(A) to 27(D) are SEM photographs of a joined structure ofmetallic members of the comparative sample of Example 1. FIG. 27(A) is awhole photograph of a joint part and its neighborhood, FIG. 27(B) is aphotograph of an upper portion of a joined interface part between anFe-based metallic member and a joining layer, FIG. 27(C) is a photographof a central portion of a joined interface part between an Fe-basedmetallic member and a joining layer, and FIG. 27(D) is a photograph of alower portion of a joined interface part between an Fe-based metallicmember and a joining layer.

FIG. 28(A) is SEM photograph (left-side photograph) of a joint part of ajoined structure of Example 2 and enlarged SEM photograph (right-sidephotograph) of an interface between an Fe-based metallic member and ajoining layer in the left-side photograph, and FIG. 28(B) is EPMA mapanalysis photographs of the interface shown in the enlarged photographof FIG. 28(A).

FIGS. 29(A) and 29(B) are SEM photographs of an interface between anFe-based metallic member and a joining layer of Example 2. FIG. 29(A) isSEM photograph of 3,000 magnifications, and FIG. 29(B) is SEM photographof 15,000 magnifications.

FIGS. 30(A) and 30(B) are schematic cross-sectional block diagrams of ajoined structure for explaining methods of a flare tensile strength testand a peel strength test in Example 2.

FIG. 31 is a graph showing strength of each sample obtained in the flaretensile strength test of Example 2.

FIG. 32 is a graph showing strength of each sample obtained in the peelstrength test of Example 2.

MODE FOR CARRYING OUT THE INVENTION First Embodiment

A first embodiment of the present invention is described below byreferring to the Drawings. FIG. 1(A) and FIG. 1(B) show the state thatjoining is conducted using a method for joining metallic membersaccording to the first embodiment. FIG. 1(A) is a schematic perspectiveview, and FIG. 1(B) is a schematic front view. FIG. 2(A) and FIG. 2(B)show an example of irradiation state of laser beam to a joined part ofmetallic members in FIG. 1(A) and FIG. 1(B). FIG. 2(A) is an enlargedfront view in the case that the center line of laser beam corresponds tothe center line of a groove shape of metallic members, and FIG. 2(B) isan enlarged front view in the case that the center line of laser beamshifts to an Fe-based metallic member side from the center line of thegroove shape of metallic members.

The method for joining metallic members uses an arrangement forproducing, for example, a flare joint. The Fe-based metallic member 1containing the Fe-based material and an Al-based metallic member 2containing the Al-based material are used as the metallic members. TheFe-based metallic member 1 and the Al-based metallic member 2 havecurved portions 11 and 12, respectively. In the arrangement of theFe-based metallic member 1 and the Al-based metallic member 2, thecurved portions 11 and 12 are faced with each other, and a groove shape13 is formed by the curved portions 11 and 12. In this case, steppedportion is provided on a facing part between the Fe-based metallicmember 1 and the Al-based metallic member 2.

In the method for joining metallic members according to the firstembodiment, a wire-shaped Zn-based brazing filler metal 3 is fed to thecentral portion of the groove shape 13 formed by the curved portions 11and 12 of the Fe-based metallic member 1 and the Al-based metallicmember 2 through a wire guide 101, and during feeding, a tip portion ofthe Zn-based brazing filler metal 3 is irradiated with laser beam 102.The Zn-based brazing filler metal 3 can be any brazing filler metal solong as Zn is a main component, and may contain or may not contain Al.In the irradiation with the laser beam 102, the joined part of theFe-based metallic member 1 is heated at a temperature higher than amelting point of the Fe-based material as its constituent material.

In this case, the center line I of the laser beam 102 may correspond tothe center line of the groove shape 13 as shown in FIG. 2(A), but it ispreferred that the center line I′ of the laser beam 102 positions at theFe-based metallic member 1 side from the center line of the groove shape13 as shown in FIG. 2(B). “I” in FIG. 2(B) shows the center line of thelaser beam in FIG. 2(A). The joined part of the Fe-based metallic member1 is heated such that a key hole is formed in the Fe-based metallicmember 1. In this case, a shielding gas is fed to the joined part,thereby shielding the joined part from the atmosphere.

The heating by the irradiation with the laser beam 102 is conducted fromthe near side to the far side in FIG. 1 along the extending direction ofthe groove shape 13. As a result, a joined structure 10 between theFe-based metallic member 1 and the Al-based metallic member 2 can beproduced as shown in FIG. 3. The path of the laser beam 102 irradiatedat the joining is shown in FIG. 3.

The joined structure 10 is provided with the Fe-based metallic member 1and the Al-based metallic member 2, and the joining layer 4 made of theZn—Al-based material, containing Zn as a main component and Al, isformed between the Fe-based metallic member 1 and the Al-based metallicmember 2. An intermetallic compound layer 5 containing an Al—Fe—Znsystem intermetallic compound containing Al as a main component isformed in the interface part between the Fe-based metallic member 1 andthe joining layer 4. The Al of the Al-based metallic member flows intothe joining layer 4 by welding. Therefore, even in the case that theZn-based brazing filler metal 3 does not contain Al, the joining layer 4and the intermetallic compound layer 5 contain Al. In this case, theintermetallic compound layer 5 is preferably formed along the entireinterface of the interface part between the Fe-based metallic member 1and joining layer 4. Furthermore, in order that the intermetalliccompound layer 5 has a stable layer shape, its composition ratio ispreferably Al: 40 to 60%, Fe: 30 to 40% and Zn: 10 to 25%. It isconsidered that the intermetallic compound layer 5 has the followingactions and effects. That is, the intermetallic compound layer 5 has theaction to suppress a reaction between Fe and Al, and it is presumed thatthis action prevents Al from flowing into the Fe-based metallic member 1and Fe from flowing in the Al-based metallic member 2.

In the first embodiment, the joined part of the Fe-based metallic member1 is heated at a temperature higher than a melting point of the Fe-basedmaterial at the joining. Therefore, the intermetallic compound layer 5made of an Al—Fe—Zn system intermetallic compound containing Al as amain component can be formed in the interface part between the Fe-basedmetallic member 1 and the joining layer 4 containing the Zn-basedbrazing filler metal 3. The intermetallic compound layer 5 has highductility, and therefore can increase joint strength between theFe-based metallic member 1 and the joining layer 4. Consequently, jointstrength between the Fe-based metallic member 1 and the Al-basedmetallic member 2 can be improved. Furthermore, by the heating at atemperature higher than a melting point of the Fe-based material asdescribed above, the Zn-based material and the Fe—Zn-based materialvaporize. By this, the plated portion plated on the Fe-based materialvaporizes regardless of the kinds of plating such as GA plating or GIplating. Consequently, good joint part can be obtained regardless of thekinds of plating. Furthermore, an oxide coating film on the surface ofthe Fe-based material is removed by vapor pressure in the melting andvaporization due to overheating. Therefore, even though flux is notused, good joining of dissimilar materials can be conducted.

Particularly, in the joining, the Fe-based material of the Fe-basedmetallic member 1 can selectively be melted by positioning the centerline I′ of the laser beam 102 at the Fe-based metallic member 1 siderelative to the center line of the groove shape 13. As a result, theintermetallic compound layer 5 can be formed into a stable layer shapeover the entire interface part between the Fe-based metallic member 1and the joining layer 4. Furthermore, the Al-based material of theAl-based metallic member 2 is not excessively heated, and this canprevent the Al-based material from melting and dropping down. Therefore,joint strength between the Fe-based metallic member 1 and the Al-basedmetallic member 2 can further be improved.

At the joining, the joined part of the Fe-based metallic member 1 isheated such that the key hole is formed in the Fe-based metallic member1. Therefore, the Zn-based brazing filler metal 3 flows into the moltenpart of the Fe-based metallic member 1 at the joining. This makes itpossible to obtain a shape that the joining layer 4 is fitted to theFe-based metallic member 1. Laser multiply reflects in the key hole.Therefore, energy density is high and temperature on the surface in thekey hole is maintained uniformly. This can uniformly form theintermetallic compound layer 5 over the upper portion, the centralportion and the lower portion of the joint part. Therefore, jointstrength between the Fe-based metallic member and the joining layer 4can further be improved, and consequently joint strength between theFe-based metallic member 1 and the Al-based metallic member 2 canfurther be improved.

The first embodiment is described in further detail by referring to thespecific examples.

In Examples 1 to 3 and Comparative Examples 1 to 3, two metallic memberswere arranged and a groove shape was formed by curved portions of thosemetallic members, as same as the arrangement embodiment shown in FIG.1(A) and FIG. 1(B). A wire-shaped Zn-based brazing filler metal was fedto a central portion of the groove shape through a wire guide, and whilefeeding, a tip portion of the Zn-based brazing filler metal wasirradiated with laser beam. Thus, a joined structure of metallic memberswas produced.

Joining conditions of Examples 1 and 2 and Comparative Examples 1 to 3are shown in Table 1. Regarding the metallic member, the indication“Fe/Al” shows that a steel plate which is the Fe-based metallic memberand an Al alloy plate which is the Al-based metallic member were used astwo metallic members. The indication “Fe/Fe” shows that the steel platewhich is the Fe-based metallic member was used as two metallic members.Regarding the material of the brazing filler metal, “ZnAl” shows that aZnAl-based brazing filler metal (containing unavoidable impurities)having a composition ratio (wt %) of Zn:Al=96:4 was used, and “Zn” showsthat a Zn-based brazing filler metal (containing unavoidable impurities)in which Al is not contained and Zn is 100 wt % was used. Regarding theposition of beam irradiation, the indication “Center” shows thearrangement that the center line of laser beam corresponds to the centerline of the groove shape of two metallic members (the arrangement ofFIG. 2(A)), and the indication “Fe side” shows that the center line oflaser beam shifts 0.6 mm to the Fe-based metallic member side from thecenterline of the groove shape (the arrangement of FIG. 2(B)).

TABLE 1 Combination of matrixes (Upper) Material of Laser Wire brazingfiller beam feeding Irradiation Joining metal Heating output speedposition of speed (Lower) state (kW) (m/min) laser beam (m/min) Example1 Fe/Al Fe 1.2 2.5 Fe side 1 ZnAl melted Example 2 Fe/Al Fe 1.2 2.5Center 1 ZnAl melted Example 3 Fe/Al Fe 1.2 2.5 Fe side 1 Zn meltedComparative Fe/Al Fe not 1   2   Center 1 Example 1 ZnAl meltedComparative Fe/Al Fe 1.6 2   Center 1 Example 2 ZnAl melted ComparativeFe/Fe Fe 1.2 2.5 Fe side 1 Example 3 ZnAl melted

Regarding other common joining conditions of Examples 1 to 3 andComparative Examples 1 to 3, a size of two metallic members was that alength in horizontal direction in FIG. 1 is 82 mm and a length inlongitudinal direction in FIG. 1 is 200 mm, and a height of the steppedportion at the joined part of two metallic members was 5 mm. Ar gas wasused as a shielding gas, and its feed amount was 25 liters/min.Irradiation angle of laser beam was 40°.

Regarding the thus-obtained joined structures of the metallic members ofExamples 1 to 3 and Comparative Examples 1 to 3, the state of the jointpart and its neighborhood was observed using a scanning electronmicroscope (SEM), and a composition ratio (atm %) of the joint part andits neighborhood was obtained using an energy dispersion X-ray analyzer(EDX analyzer). Then, joint strength of the joined structures ofExamples 1 to 3 and Comparative Examples 1 to 3 was obtained. Theresults are shown in FIGS. 4 to 9 and Table 2.

FIGS. 4 to 9 are SEM photographs of the joined structures of metallicmembers of Examples 1 to 3 and Comparative Examples 1 to 3. In eachdrawing, (A) is a whole photograph of the joint part and itsneighborhood, (B) is a photograph of an upper portion P of a joinedinterface part between the Fe-based metallic member and the joininglayer in (A), (C) is a photograph of a central portion Q in a joinedinterface part between the Fe-based metallic member and the joininglayer, and (D) is a photograph of a lower portion R of a joinedinterface part between the Fe-based metallic member and the joininglayer. In Table 2, regarding the judgment of strength, strength of theAl-based metallic member itself used in the Examples and the ComparativeExamples is about 240 N/mm, and joint strength between the Al-basedmetallic members is about 140 N/mm. Therefore, in the case that jointstrength in each of the Examples and the Comparative Examples is about140 N/mm or more, its strength was indicated as Good: A. The mark “A” inthe evaluation means “Good”, “B” means “Problem”, and “C” means

“Poor”.

TABLE 2 State of joined interface part (intermetallic compound layer)between Fe-based metallic member and joining layer Composition ratioJoint State of joined Al Fe Zn strength Judgment Site interface (atm %)(atm %) (atm %) (N/mm) of strength Example 1 Upper Stable layer shape A52 29 19 154 A Central Stable layer shape A 52 29 19 Lower Stable layershape A 52 29 19 Example 2 Upper Layer shape having B 37 46 17 96 Cunclear boundary Central Stable layer shape A 41 40 19 Lower Layer shapehaving B 18 19 63 unclear boundary Example 3 Upper Stable layer shape A60 21 19 149 A Central Stable layer shape A 58 23 19 Lower Stable layershape A 61 28 9 Comparative Upper Stable layer shape A 40 37 23 37 CExample 1 Central Layer shape having B 25 35 40 unclear boundary LowerNo intermetallic C None None None compound layer Comparative UpperDendrite shape C 48 30 22 30 C Example 2 (No boundary surface) CentralDendrite shape C 59 26 15 (No boundary surface) Lower Mottled shape C 309 61 Comparative Upper Mottled shape C 70 15 15 56 C Example 3 CentralMottled shape C 65 15 20 Lower Meander shape C 70 20 10

In the joining of Example 1, as shown in Table 1, a Fe-based metallicmember and an Al-based metallic member were used as two metallicmembers, and appropriate heat input conditions in which a joined part ofthe Fe-based metallic member is appropriately melted by heating wereemployed. In the laser beam irradiation, the center line of laser beamwas shifted 0.6 mm to the Fe-based metallic member side from the centerline of a groove shape of two metallic members. In the joined structureof Example 1 obtained by the joining conditions, as shown in FIG. 4 andTable 2, the intermetallic compound layer made of the Al—Fe—Zn systemintermetallic compound having a stable layer shape was formed in all ofthe upper portion P, the central portion Q and the lower portion R ofthe joined interface part between the Fe-based metallic member and thejoining layer, and the layer was mostly occupied by a compound having acomposition ratio of Al:Fe:Zn=52:29:19 (about 5:3:2). The compositionratio obtained by SEM analysis was Al:Fe:Zn=57:30:13. Joint strength ofthe joined structure of Example 1 was 154 N/mm.

In the joining of Example 2, as shown in Table 1, a Fe-based metallicmember and an Al-based metallic member were used as two metallicmembers, and appropriate heat input conditions in which a joined part ofthe Fe-based metallic member is appropriately melted by heating wereemployed. In the laser beam irradiation, the center line of laser beamcorresponded to the center line of a groove shape of two metallicmembers. In the joined structure of Example 2 obtained by the joiningconditions, as shown in FIG. 5 and Table 2, the boundary surface betweenthe intermetallic compound layer made of the Al—Fe—Zn systemintermetallic compound and the joining layer was unclear in the upperportion P and the lower portion R of the joined interface part betweenthe Fe-based metallic member and the joining layer as compared with thejoined structure of Example 1, but the intermetallic compound layer madeof the Al—Fe—Zn system intermetallic compound having a stable layershape was formed in the central portion Q of the joined interface partbetween the Fe-based metallic member and the joining layer. Theintermetallic compound layer was mostly occupied by a compound having acomposition ratio of Al:Fe:Zn=41:40:19. Joint strength of the joinedstructure of Example 2 was 96 N/mm.

In the joining of Example 3, as shown in Table 1, joining conditionswere set to the same joining condition as in Example 3, except for usingan Al-free Zn-based brazing filler metal as a brazing filler metal, andlaser beam irradiation was conducted. In a joined structure of Example 1obtained by the joining conditions, as shown in FIG. 6 and Table 2, theintermetallic compound layer made of the Al—Fe—Zn system intermetalliccompound having a stable layer shape was formed in all of the upperportion P, the central portion Q and the lower portion R of the joinedinterface part between the Fe-based metallic member and the joininglayer, and the layer was mostly occupied by a compound having acomposition ratio of Al:Fe:Zn=58:23:19. Joint strength of the joinedstructure of Example 3 was 149 N/mm.

In the joining of Comparative Example 1, as shown in Table 1, a Fe-basedmetallic member and an Al-based metallic member were used as twometallic members, insufficient heat input conditions in which a joinedpart of the Fe-based metallic member is not melted by heating wereemployed, and in the laser beam irradiation, the center line of laserbeam corresponded to the center line of a groove shape of two metallicmembers. In the joined structure of Comparative Example 1 obtained bythe joining conditions, as shown in FIG. 7 and Table 2, theintermetallic compound layer made of the Al—Fe—Zn system intermetalliccompound having a stable layer shape was formed in the upper portion Pof the joined interface part between the Fe-based metallic member andthe joining layer. However, the boundary surface between theintermetallic compound layer made of the Al—Fe—Zn-based intermetalliccompound and the joining layer was unclear in the central portion Q ofthe joined interface part between the Fe-based metallic member and thejoining layer, and an intermetallic compound layer was not formed in thelower portion R of the joined interface part. Joint strength of thejoined structure of Comparative Example 1 was 37 N/mm.

In the joining of Comparative Example 2, as shown in Table 1, a Fe-basedmetallic member and an Al-based metallic member were used as twometallic members, excessive heat input conditions in which a joined partof the Fe-based metallic member is melted by excessively heating wereemployed, and in the laser beam irradiation, the center line of laserbeam corresponded to the center line of a groove shape of two metallicmembers. In the joined structure of Comparative Example 2 obtained bythe joining conditions, as shown in FIG. 8 and Table 2, theintermetallic compound layer made of the intermetallic compoundcontaining Al, Fe and Zn was formed in the upper portion P, the centralportion Q and the lower portion R of the joined interface part betweenthe Fe-based metallic member and the joining layer. The intermetalliccompound layer had a dendrite shape (no boundary surface) in the upperportion P, and a mottled shape (no boundary surface at joining layerside) in the central portion Q4 and the lower portion R. Joint strengthof the joined structure of Comparative Example 2 was 30 N/mm.

In the joining of Comparative Example 3, as shown in Table 1, a Fe-basedmetallic member was used as both of two metallic members, appropriateheat input conditions in which the joined part of the Fe-based metallicmember is melted by appropriately heating were employed, and in thelaser beam irradiation, the center line of laser beam was shifted 0.6 mmto the Fe-based metallic member side from the center line of a grooveshape of two metallic members. In the joined structure of ComparativeExample 3 obtained by the joining conditions, as shown in FIG. 9 andTable 2, the intermetallic compound layer made of the intermetalliccompound containing Al, Fe and Zn was formed in the upper portion P, thecentral portion Q and the lower portion R of the joined interface partbetween the Fe-based metallic member and the joining layer. Theintermetallic compound layer had a mottled shape (no boundary surface atjoining layer side) in the upper portion P and the central portion Q,and a meander shape in the lower portion R. Joint strength of the joinedstructure of Comparative Example 3 was 56 N/mm.

As described above, in the joined structures of Examples 1 to 3, it wasconfirmed that by using the Fe-based metallic member and the Al-basedmetallic member as metallic members and using the heat input conditionsin which Fe is appropriately melted, the intermetallic compound layermade of the Al—Fe—Zn system intermetallic compound can be formed in allof the upper portion P to the lower portion R of the joined interfacepart between the Fe-based metallic member and the joining layer andjoint strength can be improved, as compared with the joined structuresof Comparative Examples 1 to 3. It was further confirmed that theintermetallic compound layer has a stable layer shape regardless of thepresence or absence of Al in the Zn-based brazing filler metal. In thecase that the intermetallic compound layer has a stable layer shape, itwas confirmed that its composition ratio is satisfied with Al: 40 to60%, Fe: 30 to 40% and Zn: 10 to 25%.

Particularly, it was confirmed in the joined structures of Examples 1and 3 that by shifting the irradiation position of laser beam to theFe-based metallic member side from the center line of the groove shape,the Al—Fe—Zn system intermetallic compound layer having a stable layershape can be formed in all of the upper portion to the lower portion ofthe joined interface part between the Fe-based metallic member and thejoining layer and joint strength can be improved, as compared with thejoined structure of Example 2. It was further confirmed that theboundary between the Fe-based metallic member and the joining layer andthe boundary between the Al-based metallic member and the joining memberbecome clear as the intermetallic compound layer becomes a stable layershape. It was understood from this fact that the intermetallic compoundlayer has the action to suppress a reaction between Fe and Al and theaction can prevent Al from flowing into the Fe-based metallic member andFe from flowing into the Al-based metallic member.

Second Embodiment

A second embodiment of the present invention is described below byreferring to the drawings. FIGS. 10(A) and 10(B) show a schematicconstitution of the state that joining is conducted using a method forjoining metallic members according to a second embodiment. FIG. 10(A) isa schematic perspective view, and FIG. 10(B) is a front view.

The method for joining metallic members uses an arrangement forproducing, for example, a flare joint. A Fe-based metallic member 1001containing a Fe-based material and an Al-based metallic member 1002containing an Al-based material are used as the metallic members. TheFe-based metallic member 1 and the Al-based metallic member 2 havecurved portions 1011 and 1012, respectively. In the arrangement of theFe-based metallic member 1001 and the Al-based metallic member 1002, thecurved portions 1011 and 1012 face each other, and a groove shape 1013is formed by the curved portions 1011 and 1012. In this case, steppedportion is provided on the faced part of the Fe-based metallic member1001 and the Al-based metallic member 1002.

In the method for joining metallic members according to the presentembodiment, a Zn—Si-based brazing filler metal 1003 having a wire-shapedis fed to the central portion of the groove shape 13 through a wireguide 1101, and while feeding, a tip portion of the Zn—Si-based brazingfiller metal 1003 is irradiated with laser beam. The Zn—Si-based brazingfiller metal 1003 containing Zn, Si and unavoidable impurities. In thiscase, it is preferred that Si is contained in an amount of 0.25 to 2.5%by weight, and the remainder contains Zn and unavoidable impurities.

In the irradiation with laser beam 1102, it is preferred that the joinedpart between the Fe-based metallic member 1001 and the Al-based metallicmember 1002 is heated at a temperature higher than a melting point ofthe Fe-based material. FIG. 11 is a cross-sectional view showing aschematic constitution of the joined part in which a keyhole 1005 isformed at the joining of the Fe-based metallic member 1001 and theAl-based metallic member 1002. In the joined part, melting andevaporation of a material occurs by heating, and the key hole 1005 isformed by evaporation reaction force (the arrow in FIG. 11) by thematerial evaporated. In this case, the molten Zn—Si-based brazing fillermetal is present on the periphery of the laser irradiation part. In thekey hole 1005, the laser beam multiply reflects as shown dotted line inthe FIG. 11. Therefore, in the key hole 1005, energy density isincreased, and the entire surface from the upper side to the lower sidein the key hole 1005 is nearly uniformly heated. By this, after passingthe laser beam 1102, the molten Zn—Si-based brazing filler metal enteredthe key hole 1005 can uniformly react with the entire surface in the keyhole 1005.

By conducting the heating by irradiation with the laser beam 1102 to thegroove shape 1013 from the near side to the far side in FIG. 10 alongthe extending direction of the groove shape 1013, a joined structure1010 of the Fe-based metallic member 1001 and the Al-based metallicmember 1002 can be produced as shown in FIG. 12.

The joined structure 1010 is provided with the Fe-based metallic member1001 and the Al-based metallic member 1002, and a joining layer 1004containing a Zn—Si-based material is formed between the Fe-basedmetallic member 1001 and the Al-based metallic member 1002. In thesecond embodiment, an intermetallic compound layer is not present in theinterface part between the Fe-based metallic member 1001 and the joininglayer 1004. In this case, in the joining layer 1004, Si particles arescattered in a matrix, and smaller particle diameter thereof ispreferred. Specifically, the particle size of Si is preferably a size(for example, 10 μm or less) which does not impair mechanical elongationpossessed by Zn. It is presumed that the formation of Si fine particlesis conducted by the cutting of crystal particles at an extrusion processin the production of a brazing filler metal.

In the second embodiment, joining of dissimilar metallic members betweenthe Fe-based metallic member 1001 and the Al-based metallic member 1002is conducted using the Zn—Si-based brazing filler metal 1003. Therefore,a brittle intermetallic compound layer is not formed in the interfacepart between the Fe-based metallic member 1001 and the Al-based metallicmember 1002. As a result, strength of the interface part between theFe-based metallic member 1001 and the joining layer 1004 can beimproved, and consequently the joined structure 1010 can obtain jointstrength nearly equal to that of the joining of similar metallicmembers. Particularly, a brazing filler metal containing 0.25 to 2.5% byweight of Si and the remainder being Zn and unavoidable impurities isused as the Zn—Si-based brazing filler metal 1003, and therefore, jointstrength (particularly peel strength) can further be improved.

The molten Zn—Si-based brazing filler metal entered the key hole 1005formed in the joined part of the Fe-based metallic member 1001 canuniformly react with the entire surface in the key hole 1005, andstrength of the interface part between the Fe-based metallic member 1001and the joining layer 1004 can further be increased. As a result, jointstrength of the joined structure 1010 can further be improved.

A Zn-based material and a Fe—Zn-based material vaporize. By this, platedportion plated on the Fe-based material vaporizes regardless of thekinds of plating such as GA plating or GI plating. Therefore, good jointpart can be obtained regardless of the kinds of plating. Furthermore, anoxide coating film on the surface of the Fe-based material is removed byvapor pressure in the melting and vaporization due to overheating.Therefore, even though flux is not used, joining of dissimilar metallicmembers can well be conducted.

The second embodiment is described in further detail below by referringto the specific example.

In the example, the Fe-based metallic member and the Al-based metallicmember were arranged as same as the arrangement embodiment shown in FIG.10, and a groove shape was formed by curved portions of those metallicmembers. A Zn—Si-based brazing filler metal having wire-shaped was fedto a central portion of the groove shape, and while feeding, a tipportion of the Zn—Si-based brazing filler metal having wire-shaped wasirradiated with laser beam. Thus, a flare joint-shaped joined structureof metallic members was produced.

Joining conditions were that a light collection diameter of laser beamis 1.8 mm, a laser output is 1.4 kV, a joining speed is 1 m/min, and awire speed is 3.2 m/min. A steel plate (JAC270, plate thickness: 1.0 mm,length in longitudinal direction in FIG. 1: 200 mm, length in horizontaldirection in FIG. 1: 80 mm) was used as the Fe-based metallic member,and an Al plate (A6K21-T14, plate thickness: 1.0 mm, length inlongitudinal direction in FIG. 1: 200 mm, length in horizontal directionin FIG. 1: 80 mm) was used as the Al-based metallic member.

In the above joining of metallic members, Zn—Si-based brazing fillermetals having different Si content (Si content is 0.25 wt %, 1.0 wt %and 2.5 wt %) are provided, joining of metallic members is conductedusing each Zn—Si-based brazing filler metal, and joined structures ofmetallic members corresponding to each of the Zn—Si-based brazing fillermetals were obtained. Each joined structure was cut into a strip in adirection perpendicular to a joining direction, and plural test pieceswere obtained. A wire diameter of all Zn—Si-based brazing filler metalswas 1.2 mm. Various evaluations were conducted using the thus-obtainedtest pieces of the joined structures.

[EPMA Elemental Map Analysis and SEM Observation of Interface PartBetween Fe-Based Metallic Member and Joining Layer]

A test piece of the joined structure obtained using the Zn—Si-basedbrazing filler metal having Si content of 1.0 wt % was subjected toelemental analysis by an electron prove microanalyzer (EPMA). Theresults obtained are shown in FIG. 13(A) and FIG. 13(B). FIG. 13(A) isSEM photograph (left-side photograph) of the joint part of the joinedstructure and enlarged SEM photograph (right-side photograph) of theinterface between the Fe-based metallic member and the joining layer inthe photograph, and FIG. 13(B) is EPMA map analysis photographs (Zn, Al,Fe and Si) of the interface shown in the enlarged SEM photograph of FIG.13(A).

Regarding a test piece of the same joined structure, the interfacebetween the Fe-based metallic member and the joining layer was observedwith a scanning electron microscope (SEM). The results obtained areshown in FIG. 14(A) and FIG. 14(B). FIG. 14(A) and FIG. 14(B) are SEMphotographs of the interface between the Fe-based metallic member andthe joining layer. FIG. 14(A) is SEM photograph of 3,000 magnifications,and FIG. 14(B) is SEM photograph of 15,000 magnifications.

As shown in FIG. 13(A), an intermetallic compound layer formed in theconventional joined structure was not observed in the interface betweenthe Fe-based metallic member and the joining layer of the presentexample. In the EPMA elemental map analysis shown in FIG. 13(B), Si wasuniformly scattered, and the interface of Fe and Zn (that is, theinterface between the Fe-based metallic member and the joining layer)was clearly observed. Al is Al of the Al-based metallic member, whichwas solid-solubilized to the joining layer by welding. Furthermore, asshown in the SEM photograph of 3,000 magnifications of FIG. 14(A) andthe SEM photograph of 15,000 magnifications of FIG. 14(B), even thoughmagnification in SEM observation was increased, an intermetalliccompound layer formed in the conventional joined structure was notobserved in the interface between the Fe-based metallic member and thejoining layer of the present example.

As a result of the above EPMA elemental map analysis and SEMobservation, it was confirmed that a brittle intermetallic compoundlayer formed in a joined structure joined with a Zn—Si-based brazingfiller metal is not present in the interface between the Fe-basedmetallic member and the joining layer of the example of the presentinvention.

[Joint Strength Evaluation of Metal Joined Structure]

A test piece of each joined structure using a Zn—Si-based brazing fillermetal having Si content of 0.25 wt %, 1.0 wt % or 2.5 wt % was subjectedto a flare tensile strength test and a peel strength test. Two pieces atthe central portion side of the joined structure and four test pieces atthe both end portion sides thereof were used as the test piece. Thosetest pieces were allocated to each strength test, and one test piece atthe central portion side and two test pieces at both end portion sides(total: three test pieces) were used in each of the flare tensilestrength test and the peel strength test.

In the flare tensile strength test, forces in mutually oppositedirections were applied to the extending portions in a horizontaldirection of the Fe-based metallic member 1021 and the Al-based metallicmember 1022 that form a T-shape at an area on which a joining layer 1023was formed, as shown in FIG. 15(A). In the flare tensile strength test,stress is most applied at the portions indicated by the arrows A and B.

The results (flare tensile strength value and rupture portion) are shownin Table 3 and FIG. 16. In Table 3, test results of test pieces ofjoined structures corresponding to the Zn—Si-based brazing filler metalshaving Si content of 0.25 wt %, 1.0 wt % and 2.5 wt % are indicated asSamples 11 to 13, respectively. Table 3 also shows the results ofComparative Samples 11 and 12. Comparative Sample 11 is a test piece ofthe joined structure between the Fe-based metallic member and theAl-based metallic member, obtained using a Zn—Al-based brazing fillermetal having an Al content of 6 wt % as the brazing filler metal.Comparative Sample 12 is a test piece of the joined structure of twoAl-based metallic members, obtained using the commercially availablebrazing filler metal as a brazing filler metal. Test pieces ofComparative Samples 11 and 12 are obtained by cutting the joinedstructures obtained into strips similarly to Samples 11 to 13. FIG. 16shows an average value of flare tensile strength of each sample andrupture portion.

Strength standard value of flare tensile strength test (dashed-dottedline of FIG. 16) is set as follows. Joint length of continuous weldingequivalent to one shot of spot welding is set to 20 mm, and spot weldingof Al each other in JIS 23140 was taken as the standard. By this,tensile strength standard of spot welding in which a plate thickness ofAl is 1.2 mm is 1.86 kN/20 mm.

TABLE 3 Combination Average of matrixes value (upper) of MaterialTensile tensile of brazing strength in strength filler metal eachportion (kN/ Rupture (lower) (kN/20 mm) 20 mm) portion Sample 11 Fe/Al2.97 3.40 3.18 3.18 Al-based ZnSi metallic (Si content: member 0.25%)(HAZ) Sample 12 Fe/Al 3.12 3.15 2.92 3.06 Al-based ZnSi metallic (Sicontent: member 1%) (HAZ) Sample 13 Fe/Al 2.96 2.86 3.06 2.96 Al-basedZnSi metallic (Si content: member 2.5%) (HAZ) Comparative Fe/Al 1.901.80 2.58 2.09 Interface Sample 11 ZnAl between (Al content: Fe-based6%) metallic member and brazing filler metal Comparative Al/Al 3.35 2.512.40 2.75 Al-based Sample 12 Commercially metallic available memberbrazing filler metal

As shown in Table 3 and FIG. 16, tensile strength of Samples 11 to 13according to the second embodiment, which are joined structures ofdissimilar metallic members, exceeded the strength standard value.Furthermore, tensile strength of Samples 11 to 13 according to thesecond embodiment was higher than tensile strength of Comparative Sample11 that is a joined structure of dissimilar metallic members, and wasalso higher than tensile strength of Comparative Sample 12 that is ajoined structure of similar metallic members. It was confirmed thattensile strength was greatly improved in a small Si content (0.25 wt %).It was further confirmed that Samples 11 to 13 according to the secondembodiment were ruptured in the Al-based metallic member, unlikeComparative Sample 11 that was ruptured in the interface between theFe-based metallic member and the joining layer.

In a peel strength test, forces in mutually opposite directions wereapplied to the extending portions in a horizontal direction of theFe-based metallic member 1021 and the Al-based metallic member 1022 thatform a T-shape at an area opposite the area on which a joining layer1023 was formed, as shown in FIG. 15(B). In the peel strength test, byconcentrating high stress to a joined interface (the portion indicatedby the arrow C), strength of joined interface can be measured.

The results (peel tensile strength value and rupture state) are shown inTable 4 and FIG. 17. In Table 4, test results of test pieces of joinedstructures corresponding to the Zn—Si-based brazing filler metals havingSi content of 0.25 wt %, 1.0 wt % and 2.5 wt % are shown as Samples 21to 23, respectively. Table 4 also shows the results of ComparativeSamples 21 and 22. Comparative Sample 21 is a test piece of the joinedstructure between the Fe-based metallic member and the Al-based metallicmember, obtained using the Zn—Al-based brazing filler metal having an Alcontent of 6 wt % as the brazing filler metal. Comparative Sample 22 isa test piece of a joined structure of two Al-based metallic members,obtained using the commercially available brazing filler metal as thebrazing filler metal. Test pieces of Comparative Samples 21 and 22 areobtained by cutting the joined structures obtained into strips similarto Samples 21 to 23. FIG. 17 also shows average values of peel strengthof each sample and rupture portion.

Strength standard value of peel strength test (dashed-dotted line ofFIG. 17) is 80% of peel strength value of a test piece of ComparativeSample 22 (joined structure of Al-based metallic members) which is ajoined structure of similar metallic members, obtained using thecommercially available brazing filler metal.

TABLE 4 Combination Average of matrixes value (upper) of Material PeelPeel of brazing strength in strength filler metal each portion (kN/Rupture (lower) (kN/20 mm) 20 mm) portion Sample 21 Fe/Al 0.64 0.69 0.660.66 Al-based ZnSi metallic (Si content: member 0.25%) (HAZ) Sample 22Fe/Al 0.67 0.75 0.67 0.70 Al-based ZnSi metallic (Si content: member 1%)(HAZ) Sample 23 Fe/Al 0.61 0.64 0.55 0.60 Interface ZnSi between (Sicontent: Fe-based 2.5%) metallic member and brazing filler metalComparative Fe/Al 0.18 0.20 0.17 0.18 Interface Sample 21 ZnAl between(Al content: Fe-based 6%) metallic member and brazing filler metalComparative Al/Al 0.75 0.69 0.7  0.71 Al-based Sample 22 Commerciallymetallic available member brazing filler metal

As shown in Table 4 and FIG. 17, peel strength of Samples 21 to 23according to the second embodiment that are joined structures ofdissimilar metallic members exceeded the strength standard. It wasconfirmed that peel strength of Samples 21 to 23 according to the secondembodiment was greatly improved in a small Si content (0.25%), from thecomparison with Comparative Sample 21 that is a joined structure ofdissimilar metallic members. It was further confirmed that Samples 21and 22 according to the second embodiment were ruptured in the Al-basedmetallic member similar to Comparative Sample 22 that is a joinedstructure of similar metallic members, unlike Comparative Sample 21which was ruptured in the interface between the Fe-based metallic memberand the joining layer. It is presumed that peel strength of Sample 23according to the second embodiment was slightly decreased by thedecrease in the joined interface width due to the decrease inwettability, as compared with Samples 21 and 22 according to the secondembodiment, and Sample 23 was ruptured in the interface between theFe-based metallic member and the joining layer.

As described above, in the samples of the present invention, which arethe joined structures of dissimilar metallic members using theZn—Si-based brazing filler metal having an Si content of 0.25 wt % to2.5 wt %, its strength exceeded the strength standard value. It wasunderstood that strength of the samples according to the secondembodiment was greatly improved in a small Si content (0.25 wt %) fromthe comparison with Comparative Samples which are the joined structuresof the same dissimilar metallic members. Particularly, it was understoodthat the samples of the present invention, which are the joinedstructure of dissimilar metallic members using the Zn—Si-based brazingfiller metal having an Si content of 0.25 wt % to 1.0 wt %, were notruptured in the interface between the Fe-based metallic member and thejoining layer, and were ruptured in the Al-based metallic member, andfrom this fact, a strong joined structure like the joined structure ofsimilar metallic members can be obtained.

The brazing method in the examples used laser. However, the brazingmethod is not limited to the use of laser, and various means can beused. Specifically, arc brazing using plasma, TIG or MIG as a heatsource, furnace brazing (joining using a preplaced brazing filler metal(sheet-shaped or rod-shaped preplaced brazing filler metal) in a heatingfurnace), high frequency brazing (heating a matrix by high frequencyinduction heating (IH)) and the like can be used.

Third Embodiment

A third embodiment of the present invention is described below byreferring to the drawings. FIG. 19(A) and FIG. 19(B) show schematicconstitution of the state that joining is conducted using a method forjoining metallic members according to a third embodiment. FIG. 19(A) isa schematic perspective view, and FIG. 19(B) is a view seen from anAl-based metallic member 2002 in FIG. 19(A).

The method for joining metallic members uses an arrangement forproducing, for example, flare joint. A Fe-based metallic member 2001containing a Fe-based material and an Al-based metallic member 2002containing an Al-based material are used as the metallic members. TheFe-based metallic member 2001 and the Al-based metallic member 2002 havecurved portions 2011 and 2012. In the arrangement of the Fe-basedmetallic member 2001 and the Al-based metallic member 2002, the curvedportions 2011 and 2012 face each other, and a groove shape 2013 isformed by those curved portions 2011 and 2012. In this case, steppedportion is provided in the faced portion between the Fe-based metallicmember 2001 and the Al-based metallic member 2002.

In the method for joining metallic members according to the thirdembodiment, a Zn-based brazing filler metal 2003 having wire-shaped isfed to the central portion of the groove shape 2013 through a wire guide2101, and while feeding, a tip portion of the brazing filler metal isirradiated with laser beam 2102. Since the Zn-based brazing filler metal2003 can obtain a sufficient joint strength than a Sn-based brazingfiller metal, the Zn-based brazing filler metal 2003 is preferred. TheZn-based brazing filler metal 2003 may contain Al and Si as additiveelements, and may not contain those. In the case that Si used as anadditive element, it is preferred that Si is contained in an amount of0.25 to 2.5 wt %, and the remainder contains Zn and unavoidableimpurities.

Heating by irradiation with the laser beam 2102 is conducted to thegroove shape 2013 from the near side to the far side in FIG. 19(A) alongthe extending direction of the groove shape 1013. FIGS. 20(A) to 20(D)and FIGS. 21(A) to (D) show the schematic constitutions of the methodfor joining metallic members according to the third embodiment. FIGS.20(A) to 20(D) are schematic views seen from the same direction as FIG.19(B) regarding each step, and FIGS. 21(A) to 21(D) are schematic viewsseen from the front of FIG. 19(A) regarding each step. In FIGS. 20(A) to20(D), the laser beam 2102 moves to the right side in the drawingsaccording to the step sequence. In FIGS. 20(A) to 20(D), indication ofthe Fe-based metallic member 2001 is omitted. In the heating by theirradiation with the layer beam 2102, it is preferred that a temperatureof the joined part between the Fe-based metallic member 2001 and theAl-based metallic member 2002 is set to a temperature higher than amelting point of the Fe-based material.

When the Zn-based brazing filler member 2003 is melted by theirradiation with the laser beam 2102 as shown in FIGS. 20(A) and 21(A),the molten Zn-based brazing filler metal 2003 drops down on the grooveshape 2013 and spreads thereon as shown in

FIGS. 20(B) and 21(B). When the laser beam 2102 moves and is located onthe molten Zn-based brazing filler metal 2003 as shown in FIGS. 20(C)and 21(C), the molten Zn-based brazing filler metal 2003 evaporates bythe direct irradiation with the laser beam 2102, and at the same time,the joined part of the Fe-based metallic member 2001 and the Al-basedmetallic member 2002 begins to melt. As a result, a key hole 2005 isformed in the joined part of the Fe-based metallic member 2001 and theAl-based metallic member 2002 as shown in FIGS. 20(D) and 21(D).

In this case, the formation of the key hole 2005 is conducted such thatthe key hole 2005 is filled with the evaporated Zn-based brazing fillermetal 2003. The Zn-based brazing filler metal 2003 other than theevaporated portion is present on the periphery of the upper end portionof the key hole 2005 as a molten material 2006 together with the moltenFe-based metallic member 2001 and Al-based metallic member 2002(moltenmetals). In the key hole 2005, the laser beam 2102 multiply reflects asshown in the dotted line in the drawings. As a result, energy density isincreased in the key hole 2005, and the entire surface of from the upperside to the lower side in the key hole 2005 is nearly uniformly heated.By this, the molten material 2006 entering the key hole 2005 afterpassing the laser beam 2102 can uniformly react with the entire surfacein the key hole 2005.

By conducting the heating by irradiation with the laser beam 2102 to thegroove shape 2013 from the near side to the far side in FIG. 19(A) alongthe extending direction of the groove shape 2013, joined structures2010A and 2010B of the Fe-based metallic member 2001 and the Al-basedmetallic member 2002 can be produced as shown in FIGS. 22(A) and 22(B).The joined structure 2010 is provided with the Fe-based metallic member2001 and the Al-based metallic member 2002, and the joining layer 2004containing a Zn-based material is formed between the Fe-based metallicmember 2001 and the Al-based metallic member 2002.

In the case that Si is not used as an additive element to the Zn-basedbrazing filler metal 2003, an uniform layer-shaped intermetalliccompound layer 2007 is formed in the interface part between the Fe-basedmetallic member 2001 and the joining layer 2004 as shown in FIG. 22(A).The intermetallic compound layer 2007 has the action to suppress areaction between Fe and Al, and it is presumed that the action preventsAl from flowing into the Fe-based metallic member 2001 and Fe fromflowing into the Al-based metallic member 2002.

In the case that Si is used as an additive element to the Zn-basedbrazing filler metal 2003, an intermetallic compound layer (reactionlayer) is not present in the interface part between the Fe-basedmetallic member 2001 and the joining layer 2004 as shown in FIG. 22(B).In this case, in the joining layer 2004, Si particles are scattered inthe matrix, and smaller particle diameter thereof is preferred.Specifically, it is preferred that the particle diameter of Si is a size(for example, 10 μm or less) which does not impair mechanical elongationpossessed by Zn.

As described above, in the third embodiment, the entire surface from theupper side to the lower side in the key hole 2005 is nearly uniformlyheated by multiple reflection of the laser beam 2102 in the key hole2005 during heating the joined part. Therefore, after heating the joinedpart, the molten material 2006 entering the key hole 2005 can uniformlyreact with the entire surface in the key hole 2005. Furthermore, joiningat low temperature becomes possible. Therefore, the molten material 2006entering the key hole 2005 can instantaneously coagulate. As a result,the interface part between the Fe-based metallic member 2001 and thejoining layer 2004 can uniformly be cooled.

Therefore, in the case that the intermetallic compound layer 2007 isformed between the Fe-based metallic member 2001 and the joining layer2004, the intermetallic compound layer 2007 has a uniform layer shape,and this can improve joint strength. In the case that the intermetalliccompound layer 2007 is not formed between the Fe-based metallic member2001 and the joining layer 2004, a brittle layer is not present andunevenness is not generated in strength distribution in the interfacepart between the Fe-based metallic member 2001 and the joining layer2004. As a result, joint strength can greatly be improved.

Formation of the key hole 2005 increases the joining area. Therefore,the above effect can well be obtained. Zn-based plating and alloyedFe—Zn-based plating vaporize. In this case, the plated portion plated onthe Fe-based material vaporizes regardless of the kinds of plating suchas GA plating and GI plating. Consequently, good joint part can beobtained regardless of the kinds of plating. An oxide coating film onthe surface of the Fe-based material is removed by vapor pressure in themelting and evaporation due to overheating. Therefore, even though fluxis not used, joining of dissimilar metallic members can well beconducted.

Fourth Embodiment

In a fourth embodiment, in place of conducting multiple reflection ofthe laser beam 2102 in the key hole 2006 formed by melting the joinedpart of the Fe-based metallic member 2001 and the Al-based metallicmember 2002 as in the third embodiment, the joined part of the Fe-basedmetallic member 2001 and the Al-based metallic member 2002 is not meltedand multiple reflection of the laser beam 2102 is conducted in a grooveshape 2013 formed by the Fe-based metallic member 2001 and the Al-basedmetallic member 2002. Other than the above, the fourth embodiment is thesame as the method for joining metallic members according to the thirdembodiment. In the fourth embodiment, the same constituent elements asin the third embodiment have the same reference numerals and signs, andthe explanation of the constituent elements having the same action as inthe third embodiment is omitted.

FIGS. 23(A) to 23(D) show schematic constitution of the method forjoining metallic members according to the fourth embodiment. FIGS. 23(A)to 23(D) are schematic views seen from the same direction as FIG. 19(B)regarding each step. In FIGS. 23(A) to 23(D), the laser beam 2102 movesto the right side in the drawings according to the step sequence.

When the Zn-based brazing filler member 2003 is melted by theirradiation with the laser beam 2102 as shown in FIG. 23(A), the moltenZn-based brazing filler metal 2003 drops down on the groove shape 2013and spreads thereon as shown in FIG. 23(B). When the laser beam 2102moves and is located on the molten Zn-based brazing filler metal 2003 asshown in FIG. 23(C), the molten Zn-based brazing filler metal 2003evaporates by the direct irradiation with the laser beam 2102.

In this case, the above operation is conducted such that the grooveshape 2013 is filled with the evaporated Zn-based brazing filler metal2003. The Zn-based brazing filler metal 2003 other than the evaporatedportion is present on the periphery of the upper end portion of thegroove shape 2013. On the surface in the groove shape 2013, the laserbeam 2102 multiply reflects as shown in the dotted line in the drawings.As a result, energy density is increased in the key hole 2005, and theentire surface from the upper side to the lower side in the groove shape2003 is nearly uniformly heated. By this, a molten material 2006entering the key hole 2005 after passing the laser 2102 can uniformlyreact with the entire surface in the key hole 2005.

By conducting the heating by irradiation with the laser beam 2102 to thegroove shape 2013 from the near side to the far side in FIG. 19(A) alongthe extending direction of the groove shape 2013, a joined structure2020 of the Fe-based metallic member 2001 and the Al-based metallicmember 2002 can be produced as shown in FIGS. 24(A) and 24(B). Thejoined structure 2010 is provided with the Fe-based metallic member 2001and the Al-based metallic member 2002, and a joining layer 2014containing a Zn-based material is formed between the Fe-based metallicmember 2001 and the Al-based metallic member 2002.

In the case that Si is not used as an additive element to the Zn-basedbrazing filler metal 2003, an uniform layer-shaped intermetalliccompound layer 2017 is formed in the interface part between the Fe-basedmetallic member 2001 and the joining layer 2014 as shown in FIG. 24(A).In the case that Si is used as an additive element to the Zn-basedbrazing filler metal 2003, an intermetallic compound layer (reactionlayer) is not present in the interface part between the Fe-basedmetallic member 2001 and the joining layer 2014 as shown in FIG. 24(B).

In the fourth embodiment, by conducting multiple reflection of the laserbeam 2102 on the surface of the groove shape 2103 formed by the Fe-basedmetallic member 2001 and the Al-based metallic member 2002, the entiresurface in the groove shape 2013 can nearly uniformly be heated. As aresult, the same effect by multiple reflection as in the thirdembodiment can be obtained.

Examples

The third embodiment and the fourth embodiment are described in moredetail by referring to the specific examples.

(1) Example 1 Case Using Zn—Al-Based Brazing Filler Metal

In Example 1, a Fe-based metallic member and an Al-based metallic memberwere arranged in the same arrangement embodiment shown in FIG. 19(A),and a groove shape was formed by curved portions of those metallicmembers. A wire-shaped Zn—Al-based brazing filler metal was fed to acentral portion of the groove shape through a wire guide, and whilefeeding, a tip portion of the Zn—Al-based grazing filler member wasirradiated with laser beam. Thus, a flare joint-shaped joined structureof metallic members was produced.

Regarding joining conditions, a size of two metallic members had alength in a horizontal direction of 82 mm in FIG. 19(A) and a length ina longitudinal direction of 200 mm in FIG. 19(A), and a height ofstepped portion in the joined part of two metallic members was 5 mm. Abrazing filler metal having a composition ratio (wt %) of Zn:Al=96:4 wasused as a Zn—Al-based brazing filler member. Ar gas was used as ashielding gas, and its feed amount was 25 liter/min. Irradiation angleof laser beam was 40°, and joining speed was 1 m/min.

In the above joining of metallic members, joined structures of metallicmembers (Sample 111, and Comparative Samples 111 and 112) were obtainedby chaining laser output and wire speed every sample and changing theformation state of the key hole every sample. In Sample 111, the laseroutput was set to 1.2 kW, and the wire feeding speed was set to 2.5m/min. Thus, appropriate heat input conditions in which the joined partof the Fe-based metallic member is melted by appropriate heating in theformation of a key hole were employed. In Comparative Sample 111, thelaser output was set to 1 kW, and the wire feeding speed was set to 2m/min. Thus, insufficient heat input conditions in which the joined partof the Fe-based metallic member is not melted by heating in theformation of a key hole were employed. In Comparative Sample 112, thelaser output was set to 1.6 kW, and the wire feeding speed was set to 2m/min. Thus, excessive heat input conditions in which the joined part ofthe Fe-based metallic member is melted by excessively heating in theformation of a key hole were employed.

Regarding joined structures of metallic members of thus obtained Sample111 and Comparative Samples 111 and 112, the state of the joint part andits neighborhood was observed using a scanning electron microscope(SEM). FIGS. 25 to 27 are SEM photographs of the joined structures ofmetallic members of Sample 111 and Comparative Samples 111 and 112. Ineach drawing, (A) is a whole photograph of the joint part and itsneighborhood, (B) is a photograph of the upper portion P of the joinedinterface part between the Fe-based metallic member and the joininglayer, (C) is a photograph of the central portion Q of the joinedinterface part between the Fe-based metallic member and the joininglayer, and (D) is a photograph of the lower portion R of the joinedinterface part between the Fe-based metallic member and the joininglayer.

As shown in FIG. 25, in the joined structure of Sample 111 in whichappropriate heat input conditions were employed to form the key hole, anintermetallic compound layer made of Al—Fe—Zn system intermetalliccompound having a stable layer-shaped was formed in all of the upperportion P, the central portion R and the lower portion Q of the joinedinterface part between the Fe-based metallic member and the joininglayer, and the boundary between the Fe-based metallic member and thejoining layer and the boundary between the Al-based metallic member andthe joining layer became clear. Joint strength of the joined structureof Sample 11 was 154 N/mm.

As shown in FIG. 26, in the joined structure of Comparative Sample 111in which insufficient heat input conditions were employed to form thekey hole, an intermetallic compound layer made of Al—Fe—Zn systemintermetallic compound having a stable layer-shaped was formed in theupper portion P of the joined interface part between the Fe-basedmetallic member and the joining layer. However, the boundary surfacebetween the intermetallic compound layer made of the Al—Fe—Zn systemintermetallic compound and the joining layer was unclear in the centralportion Q of the joined interface part between the Fe-based metallicmember and the joining layer, and an intermetallic compound layer wasnot formed in the lower portion R of the joined interface part. Jointstrength of the joined structure of Comparative Sample 111 was 37 N/mm.

As shown in FIG. 27, in the joined structure of Comparative Sample 112in which excessive heat input conditions were employed to form a keyhole, an intermetallic compound layer made of an intermetallic compoundcontaining Al, Fe and Zn was formed in the upper portion P, the centralportion R and the lower portion Q of the joined interface part betweenthe Fe-based metallic member and the joining layer. However, theintermetallic compound layer had a dendrite shape (no boundary surface)in the upper portion P and a mottled shape (no boundary surface atjoining layer side) in the central portion Q and the lower portion R.Joint strength of the joined structure of Comparative Sample 112 was 30N/mm.

As described above, it was confirmed in the joined structure of Sample111 that by the multiple reflection of laser beam under appropriate heatinput conditions in the formation of a key hole, an uniform layer-shapedintermetallic compound layer can be formed in all of the upper portion Pto the lower portion R of the joined interface part between the Fe-basedmetallic member and the joining layer, and joint strength can beimproved, as compared with the joined structures of Comparative Examples111 and 112. Particularly, it was confirmed that the boundary betweenthe Fe-based metallic member and the joining layer and the boundarybetween Al-based metallic member and joining layer become clear as theintermetallic compound layer becomes a stable layer shape. It wasunderstood by this that the intermetallic compound layer has the actionto suppress a reaction between Fe and Al, and the action can prevent Alfrom flowing into the Fe-based metallic member and Fe from flowing intothe Al-based metallic member.

(2) Example 2 Case Using Zn—Si-Based Brazing Filler Metal

In Example 2, a flare joint-shaped joined structure of metallic memberswas produced in the same manner as in Example 1, except for using aZn—Si-based brazing filler metal as a Zn-based brazing filler metal.Joining conditions were that a light collection diameter of laser beamis 1.8 mm, a laser output is 1.4 kV, a joining speed is 1 m/min, and awire speed is 3.2 m/min. Appropriate heat input conditions in which thejoined part of the Fe-based metallic member is melted by appropriatelyheating in the formation of a key hole were employed. A steel plate(JAC270, plate thickness: 1.0 mm, length in longitudinal direction inFIG. 19(A): 200 mm, length in horizontal direction in FIG. 19(A): 80 mm)was used as the Fe-based metallic member, and an Al plate (A6K21-T14,plate thickness: 1.0 mm, length in longitudinal direction in FIG. 19(A):200 mm, length in horizontal direction in FIG. 19(A): 80 mm) was used asthe Al-based metallic member.

In the above joining of metallic members, Zn—Si-based brazing fillermetals having different Si content (Si content is 0.25 wt %, 1.0 wt %and 2.5 wt %) are provided, joining of metallic members is conductedusing each Zn—Si-based brazing filler metal, and joined structures ofmetallic members corresponding to each of the Zn—Si-based brazing fillermetals were obtained. Each joined structure was cut into a strip in adirection perpendicular to a joining direction, and plural test pieceswere obtained. A wire diameter of all Zn—Si-based brazing filler metalswas 1.2 mm. Various evaluations were conducted using the thus-obtainedtest pieces of the joined structures.

[EPMA Elemental Map Analysis and SEM Observation of Interface PartBetween Fe-Based Metallic Member and Joining Layer]

A test piece of the joined structure obtained using the Zn—Si-basedbrazing filler metal having Si content of 1.0 wt % was subjected toelemental analysis by an electron prove microanalyzer (EPMA). Theresults obtained are shown in FIGS. 28(A) and 28(B). FIG. 28(A) is SEMphotograph (left-side photograph) of the joint part of the joinedstructure and enlarged SEM photograph (right-side photograph) of theinterface between the Fe-based metallic member and the joining layer inthe photograph, and FIG. 28(B) is EPMAmap analysis photographs (Zn, Al,Fe and Si) of the interface shown in the enlarged SEM photograph of FIG.28(A).

Regarding a test piece of the same joined structure, the interfacebetween the Fe-based metallic member and the joining layer was observedwith a scanning electron microscope (SEM). The results obtained areshown in FIGS. 29(A) and 29(B). FIGS. 29(A) and 29(B) are SEMphotographs of the interface between the Fe-based metallic member andthe joining layer. FIG. 29(A) is SEM photograph of 3,000 magnifications,and FIG. 29(B) is SEM photograph of 15,000 magnifications.

As shown in FIG. 28(A), an intermetallic compound layer was not observedin the interface between the Fe-based metallic member and the joininglayer of Example 2. In the EPMA elemental map analysis shown in FIG.28(B), Si was uniformly scattered, and the interface of Fe and Zn (thatis, the interface between the Fe-based metallic member and the joininglayer) was clearly observed. Al is Al of the Al-based metallic member,which was solid-solubilized to the joining layer by welding.Furthermore, as shown in the SEM photograph of 3,000 magnifications ofFIG. 29(A) and the SEM photograph of 15,000 magnifications of FIG.29(B), even though magnification in SEM observation was increased, anintermetallic compound layer was not observed in the interface betweenthe Fe-based metallic member and the joining layer of the presentexample.

As is understood from the results of the above EPMA elemental mapanalysis and SEM observation, it was confirmed that a brittleintermetallic compound layer formed in the conventional joined structure(joined structure joined with a Zn—Si-based brazing filler metal) is notpresent in the interface between the Fe-based metallic member and thejoining layer when laser beam multiply reflects under the appropriateheat input conditions in the formation of a key hole in the same manneras in Example 1 except for using the Zn—Si-based brazing filler metal asthe brazing filler metal.

[Joint Strength Evaluation of Metal Joined Structure]

A test piece of each joined structure using a Zn—Si-based brazing fillermetal having Si content of 0.25 wt %, 1.0 wt % or 2.5 wt % was subjectedto a flare tensile strength test and a peel strength test. Two pieces atthe central portion side of the joined structure and four test pieces atthe both end portion sides thereof were used as the test piece. Thosetest pieces were allocated to each strength test, and one test piece atthe central portion side and two test pieces at both end portion sides(total: three test pieces) were used in each of the flare tensilestrength test and the peel strength test.

In the flare tensile strength test, forces in mutually oppositedirections were applied to the extending portions in a horizontaldirection of a Fe-based metallic member 2021 and an Al-based metallicmember 2022 that form a T-shape at an area on which a joining layer 2023was formed, as shown in FIG. 30(A). In the flare tensile strength test,stress is most applied at the portions indicated by the arrows A and B.

The results (flare tensile strength value and rupture portion) are shownin Table 5 and FIG. 31. In Table 5, test results of test pieces ofjoined structures corresponding to the Zn—Si-based brazing filler metalshaving Si content of 0.25 wt %, 1.0 wt % and 2.5 wt % are indicated asSamples 21 to 23, respectively. Table 5 also shows the results of Sample24 and Comparative Samples 21. Sample 24 is a test piece of the joinedstructure of the Fe-based metallic member and the Al-based metallicmember, obtained using a Zn—Al-based brazing filler metal containing Alcontent of 6 wt %, and corresponds to the joined structure of Sample 111of Example 1. Comparative Sample 21 is a test piece of the joinedstructure of two Al-based metallic members, obtained using thecommercially available brazing filler metal as a brazing filler metal.Test pieces of Sample 24 and Comparative Sample 21 are obtained bycutting the joined structures obtained into strips similarly to Samples21 to 23. FIG. 31 shows an average value of flare tensile strength ofeach sample and rupture portion.

Strength standard value of flare tensile strength test (dashed-dottedline of FIG. 31) is set as follows. Joint length of continuous weldingequivalent to one shot of spot welding is set to 20 mm, and spot weldingof Al each other in JIS 23140 was taken as the standard. By this,tensile strength standard of spot welding in which a plate thickness ofAl is 1.2 mm is 1.86 kN/20 mm.

TABLE 5 Combination of matrixes Average (upper) Tensile value Materialstrength of tensile of brazing in each strength filler metal portion(KN/ Rupture (lower) (KN/20 mm) 20 mm) portion Sample 21 Fe/Al 2.97 3.403.18 3.18 Al-based ZnSi metallic (Si content: member 0.25%) (HAZ) Sample22 Fe/Al 3.12 3.15 2.92 3.06 Al-based ZnSi metallic (Si content: member1%) (HAZ) Sample 23 Fe/Al 2.96 2.86 3.06 2.96 Al-based ZnSi metallic (Sicontent: member 2.5%) (HAZ) Sample 24 Fe/Al 1.90 1.80 2.58 2.09Interface ZnAl between (Al content: Fe-based 6%) metallic member andbrazing filler metal Com- Al/Al 3.35 2.51 2.40 2.75 Al-based parativeCommercially metallic Sample 21 available member brazing filler metal

As shown in Table 5 and FIG. 31, tensile strength of Samples 21 to 23according to the present invention that are joined structures ofdissimilar metallic members exceeded the strength standard value.Furthermore, tensile strength of Samples 21 to 23 was higher thantensile strength of Sample 24 which is a joined structure of dissimilarmetallic members, and was also higher than tensile strength ofComparative Sample 21 which is a joined structure of similar metallicmembers. It was confirmed that tensile strength was greatly improved ina small Si content (0.25 wt %). It was further confirmed that Samples 21to 23 were ruptured in the Al-based metallic member, unlike Sample 24which was ruptured in the interface between the Fe-based metallic memberand the joining layer.

In the peel strength test, forces in mutually opposite directions wereapplied to the extending portions in a horizontal direction of theFe-based metallic member 2021 and the Al-based metallic member 2022 thatform a T-shape at an area opposite the area on which the joining layer2023 was formed, as shown in FIG. 30(B). In the peel strength test, byconcentrating high stress to a joined interface (the portion indicatedby the arrow C), strength of joined interface can be measured.

The results (peel tensile strength value and rupture state) are shown inTable 6 and FIG. 32. In Table 6, test results of test pieces of joinedstructures corresponding to the Zn—Si-based brazing filler metals havingSi content of 0.25 wt %, 1.0 wt % and 2.5 wt % are shown as Samples 31to 33, respectively. Table 6 also shows the results of Sample 34 andComparative Sample 31. Sample 34 is a test piece of the joined structureof the Fe-based metallic member and the Al-based metallic member,obtained using the Zn—Al-based brazing filler metal containing Alcontent of 6 wt %, and corresponds to the joined structure of Sample 111of Example 1. Comparative Sample 31 is a test piece of a joinedstructure of two Al-based metallic members, obtained using thecommercially available brazing filler metal as the brazing filler metal.Test pieces of Sample 34 and Comparative Sample 31 are obtained bycutting the joined structures obtained into strips, similarly to Samples31 to 33. FIG. 32 shows an average value of peel strength of each sampleand rupture portion.

Strength standard value of peel strength test (dashed-dotted line ofFIG. 32) is 80% of peel strength value of a test piece of ComparativeSample 31 (joined structure of Al-based metallic members) which is ajoined structure of similar metallic members, obtained using thecommercially available brazing filler metal.

TABLE 6 Combination Average of matrixes value (upper) of Material ofpeel brazing Peel strength in strength filler metal each portion (KN/Rupture (lower) (KN/20 mm) 20 mm) portion Sample 31 Fe/Al 0.64 0.69 0.660.66 Al-based ZnSi metallic (Si content: member 0.25%) (HAZ) Sample 32Fe/Al 0.67 0.75 0.67 0.70 Al-based ZnSi metallic (Si content: member 1%)(HAZ) Sample 33 Fe/Al 0.61 0.64 0.55 0.60 Interface ZnSi between (Sicontent: Fe-based 2.5%) metallic member and brazing filler metal Sample34 Fe/Al 0.18 0.20 0.17 0.18 Interface ZnAl between (Al content:Fe-based 6%) metallic member and brazing filler metal Comparative Al/Al0.75 0.69 0.7  0.71 Al-based Sample 31 Commercially metallic availablemember brazing filler metal

As shown in Table 5 and FIG. 32, peel strength of Samples 31 to 33 thatare joined structures of dissimilar metallic members exceeded thestrength standard. It was confirmed that peel strength of Samples 31 to33 was greatly improved in a small Si content (0.25%), from thecomparison with Sample 34 which is a joined structure of dissimilarmetallic members. It was further confirmed that Samples 31 and 32 wereruptured in the Al-based metallic member similarly to Comparative Sample31 which is a joined structure of similar metallic members, unlikeComparative Sample 34 which was ruptured in the interface between theFe-based metallic member and the joining layer. It is presumed that peelstrength of Sample 33 was slightly decreased by the decrease in thejoined interface width due to the decrease in wettability, as comparedwith Samples 31 and 32, and Sample 33 was ruptured in the interfacebetween the Fe-based metallic member and the joining layer.

As described above, in the samples which are the joined structures ofdissimilar metallic members using the Zn—Si-based brazing filler metalhaving an Si content of 0.25 wt % to 2.5 wt %, its strength exceeded thestrength standard value. It was understood that strength was greatlyimproved in a small Si content (0.25 wt %) from the comparison withComparative Samples which are the joined structures of the samedissimilar metallic members. Particularly, it was understood that thesamples which are the joined structure of dissimilar metallic membersusing the Zn—Si-based brazing filler metal having an Si content of 0.25wt % to 1.0 wt % were not ruptured in the interface between the Fe-basedmetallic member and the joining layer, and were ruptured in the Al-basedmetallic member, and from this fact, a strong joined structure like thejoined structure of similar metallic members can be obtained.

Although the present invention has been described in detail and byreference to the specific embodiments, it is apparent to one skilled inthe art that various modifications or changes can be made withoutdeparting the spirit and scope of the present invention.

INDUSTRIAL APPLICABILITY

The present invention can be utilized in a method for joining metallicmembers, comprising joining the Fe-based metallic member and theAl-based metallic member by interposing the brazing filler metal betweenthe Fe-based metallic member and the Al-based metallic member, thejoined structure and the brazing filler metal.

Description of Reference Numerals and Sings

-   -   1 Fe-based metallic member    -   2 Al-based metallic member    -   3 Zn-based brazing filler metal    -   4 Joining layer    -   5 Intermetallic compound layer    -   1001 Fe-based metallic member    -   1002 Al-based metallic member    -   1003 Zn-based brazing filler metal    -   1004 Joining layer    -   1005 Key hole    -   2001 Fe-based metallic member    -   2002 Al-based metallic member    -   2003 Zn-based brazing filler metal    -   2004, 2014 Joining layer    -   2005 Key hole    -   2006 Molten material    -   2007, 2017 Intermetallic compound layer    -   2010, 2020 Joined structure    -   2013 Groove shape    -   2102 Laser beam

1. A joined structure of metal members, comprising: a Fe-based metallicmember containing a Fe-based material and an Al-based metallic membercontaining an Al-based material, joined with a brazing filler metalconsisting of Zn, Si and unavoidable impurities interposed therebetween.2. The joined structure of metal members according to claim 1, whereinthe brazing filler metal contains 0.25 to 2.5% by weight of Si, and theremainder being Zn and unavoidable impurities.